Generally speaking, pipelines are formed from sections of steel pipe having factory-applied corrosion protection and insulating coatings. In a typical construction, the pipes are provided with at least two functional layers; an inner corrosion protection coating comprised of for example fusion-bonded epoxy (“FBE”), and an outer layer formed from a thermoplastic such as polypropylene or polyethylene. The insulating layer may be foamed or unfoamed.
In the manufacture of coated/insulated pipe, the ends of the pipe must be left bare so as to prevent damage to the coating when the pipes are joined in the field by welding. Typically, the coating is cut back from the end of the pipe to form a chamfer which is spaced from the end of the pipe. A lip of the epoxy undercoating may protrude beyond the end (or “toe”) of the chamfer. The chamfering step is typically performed in the factory as part of the manufacturing process.
The individual pipe sections are joined together in the field to form a continuous pipeline. The joints between the pipe sections are known as “field joints”, and are formed by butt welding the pipe sections together, and then applying a layer of coating/insulation over the bare pipe surrounding the weld joint. These steps may be performed as the pipeline is being reeled onto or from a lay vessel (so called “tie-in joints”), during pre-fabrication of multi-jointed pipe strings, or immediately before laying of the pipeline. For reasons of economy, field joints must be rapidly formed and cooled to an acceptable temperature so as not to slow down the reeling or laying operation. For example, reeling of the pipeline is generally not permitted unless the temperature of the field joint and associated steel is below about 100 degrees Celsius, typically below about 80 degrees Celsius.
There are numerous methods for formation of field joints. In one method known to the inventors, a corrosion protection coating of FBE is provided over the bare pipe surrounding the weld joint. The joint area is then heated to about 180-250 degrees Celsius to cure the FBE, and the insulating/coating layer is then applied over the heated joint area by injection molding.
Heating of the pipe joint to cure the FBE causes portions of the existing, factory-applied FBE coating to be heated above its glass transition temperature. The glass transition temperature is the temperature at which the FBE transforms from a hard state to a softened, rubber-like state, and is in the range from about 100 to 160 degrees Celsius. More typically, the glass transition temperature of commonly used low temperature FBEs is in the range from 100-110 degrees Celsius. The inventors have found that heating the FBE coating above its glass transition temperature can weaken the bond between the factory-applied FBE coating and the steel pipe in the region of the chamfer toe. This results in the formation of a discontinuous structure under the chamfer toe which may fail during reeling or subsequent laying of the pipeline.
Another disadvantage of this method is that a significant amount of time is required to preheat the pipe to 180-250 degrees Celsius and then cool the pipe and the applied field joint insulation system to 100 degrees Celsius or lower. Where the pipeline is being reeled onto a lay barge, for example, this additional heating and cooling time is costly as it slows the reeling process and increases lay vessel wait time.
Therefore, the need exists for a method for applying field joints to insulated pipelines which avoids excessive heating of the pipe joint area.